1. Field of the Invention
The object of the invention is a heat exchanger for transferring heat between gas flowing essentially in counter current, the walls of the heat exchanger separating the flows from each other being made of flexible film, through which the heat exchange takes place.
2. Description of the Related Art
Large amounts of energy are lost in connection of ventilation in buildings and industry. Energy is also lost in connection with the processing of various hot flue and process gases, when heat is not recovered from the gases in a sufficiently efficient manner. A typical example is a drying process, from which moist gases are removed. Large amounts of energy could be recovered especially from these, if it were also possible to utilise the energy released by condensation. This is also significant in the protection of the environment, when substances harmful or hazardous to the environment are removed from the gas flow. The removing of water vapour from a gas flow furthermore reduces the so-called visible environmental nuisance. Problems are caused by high investment costs, especially in a case in which corrosive substances are condensed from the gases, and also by the fact that structures generally available on the market operate according to the so-called cross flow principle so that at most about 60-70% of heat is recovered. Heat exchange structures for large amounts of gas are bulky and heavy, so they are difficult to transport and install.
Available in the market are plastic gasxe2x80x94gas heat exchangers, but they are characterised by having a rigid structure. Most of these operate according to the cross flow principle. Really low cost structures are not available. The introduction of a low cost heat exchanger in the market could considerably increase the recovery of heat in many applications at the same time as it could be possible to take into account environmental requirements relating to air protection in a new way.
The patent specification U.S. Pat. No. 4,411,310 discloses a method for manufacturing a heat exchanger from plastic films by bonding parallel films together at certain intervals. Plastic film pairs thus formed are connected to form a heat exchanger. The heat exchange passageways are formed when air, for example, is led into them. The patent suggests that an expanding element can be placed in the channels keeping the channels open. No permanent deformation occurs in the channels, and the dimensions of the heat exchanger change during expansion. This phenomenon often leads to cumbersome inlet and outlet organs for gases, because such a heat exchanger requires a method by which the shrinking of the element can be controlled in the expansion situation.
The patent specification WO 91/04451 discloses a heat exchanger consisting of films for heat exchange between two air flows. The heat exchanger has a separate wall structure for closing the space outside the channels. The channels are kept open by a support block. Stretching the films continuously when directing a pressurised air flow into the channel will eventually lead to undesired elongation or creep. However, the films are not stretched during the manufacturing step to cause permanent deformation. In addition, the heat exchanger has a spring structure with which tension can be adjusted. This makes the structure heavy and expensive.
The patent specification FI 970227 in the name of the present applicant discloses a heat exchange element for a film evaporator or distiller. The film elements are formed from opposite plastic films which are bonded at selected points one to another to channel the interior of the element. In connection of the manufacture, a permanent deformation is caused in the flexible film forming the heat exchange surfaces of the element by stretching it between the bonding points so that, during the later pressurisation of the element, its interior can be expanded so that it simultaneously retains its vertical and horizontal outer dimensions. The heat exchange element of the patent application FI 970227 is used for transferring heat from the vapour to be condensed inside the elements to the liquid to be evaporated on the outer surfaces of the elements.
The said problems in the heat transfer between two gases can be solved by building a heat exchange apparatus according to the invention operating with the counter current principle from flexible plastic film, the apparatus being herein referred to as xe2x80x9cheat exchangerxe2x80x9d. The separate heat exchange elements of the heat exchanger according to the invention, herein referred to as xe2x80x9celementxe2x80x9d, are built from flexible and thin plastic film.
The heat exchanger according to the invention is characterised in that the element of the heat exchanger consists of two plastic films and a support film positioned between these films, jointed together so that parallel flow channels are produced; the said channels can be expanded so that the element retains its vertical and horizontal outer dimensions; and in that several elements are connected to form a heat exchanger so that the free edges of the plastic films of adjacent elements are bonded together, whereby a closed space is formed also outside the elements; and in that the gas flow is directed to the interior of the flow channels with a higher pressure than to the exterior for expanding the flow channels.
The thickness of the plastic films used for forming the flow channels is 0.01-0.5 mm, most preferably 0.05-0.1 mm.
The support film is preferably made of a stronger plastic film than the plastic films forming the flow channel. The support film may also be made of some other reinforcing material, such as fibreglass mat or net or woven fabric. The purpose of the support film is to keep the element straight as it is in its unpressurised state and to support the element when gas is blown into the flow channels and the flow channels expand. Due to the support film, the vertical and horizontal outer dimensions of the element are retained unchanged irrespective of whether the flow channels contain gas or not.
In an advantageous embodiment of the invention, the flow channels are formed by attaching the plastic films to the support film by a continuous weld joint, and the part of the plastic film remaining between the joints is stretched so that a permanent deformation takes place in it. Gas directed to the interior of the element can flow from one end of the element to the other in the channels thus formed.
Any method presented, for example, in the patent application FI 970227, or other state-of-the-art method can be used in stretching to deform the plastic film. One such method comprises, for example, heating the plastic film and stretching it simultaneously with the aid of the pressure difference.
The flow channel can also be formed so that flexible plastic is fastened to both sides of the support film in a wavelike manner as in a corrugated board. Further, besides bonding, also spot welds or short linear or wavelike weld joints can be used, whereby a bed-like element is formed in which the flow channels are in contact with each other.
The ends of the elements have an aperture structure, which is herein referred to as xe2x80x9cbattenxe2x80x9d, through which gas is directed to flow into or out of the element.
Several elements formed in the way described above are assembled to form a heat exchanger so that the free sides of the flexible plastic films remaining in the free sides of superimposed elements are connected to each other. Thus, a heat exchanger is produced from several superimposed elements, in which also the space outside the flow channels is a closed space.
The arrangement described above reduces the manufacturing costs of the heat exchanger, as no separate side wall is required. Further, due to the flexible structure and the lack of rigid walls, the heat exchanger can be packed into a small space, which facilitates transport and installation, thus also reducing costs.
A substantially similar xe2x80x9cbattenxe2x80x9d as in the ends of the elements is placed between the elements in the end of the heat exchanger formed in the way described above. The gas flow is directed to the external space of the elements or out of the external space of the elements through this batten between the elements.
The elements may be positioned at an optional distance from each other, whereby the external space of the flow channels is larger or smaller, and the elements are arranged either so that they touch each other or so that, even when expanded, they remain separate from each other. This distance can be adjusted, for example, by changing the thickness of the batten. In different layers of the heat exchanger, the flow channels of the elements can be positioned either at corresponding points with respect to each other, or overlapping each other.
The gas flow is directed to the interior of the element with a higher pressure than to the exterior to expand the elements. The gas flows are directed to the heat exchanger preferably from its opposite ends so that the gas flows pass through the heat exchanger essentially in counter current.
The heat exchanger can be installed both in vertical or horizontal positions or in an angle between these positions. If liquid condenses from one of the gas flows, it is preferred to install the heat exchanger in a vertical position and to lead said gas flow into the heat exchanger from the upper end.
The invention is next described in greater detail by an example, referring to the accompanying drawings, in which